The present invention relates generally to an in-line centrifugal fan, and in particular, relates to a mixed flow fan having a high operating efficiency and reduced sound output, and that is easy to manufacture and service.
In-line fans are generally classified according to the direction of airflow through the impeller. In particular, axial flow fans are characterized by flow through the impeller in a direction generally parallel to the shaft axis. In-line centrifugal fans receive airflow into the impeller axially, and redirect the airflow radially outward. Mixed flow fans are characterized in that the air enters the impeller axially and is deflected at an obtuse angle by the impeller blades such that the air flowing out of the impeller has both axial and radial flow components.
The performance and desirability of a fan is measured generally by the fan's efficiency and sound levels produced during operation. The optimization of these two components will reduce the energy needed to operate the fan, thus conserving cost, and will further reduce the noise pollution associated with operation as frequent exposure to high levels of noise pollution has been linked to various health problems in humans and is generally annoying. One leading mixed flow fan in the industry was commercially introduced in 1997 as the leading fan in the industry in terms of high efficiency and low sound levels. This fan was tested in accordance with standards adopted by the Air Movement and Control Association to determine the fan's efficiency and sound power output under various operating conditions, such as fan static pressure (water gauge) and flow rate, measured in cubic feet per minute (CFM). The sound pressure level was reported in dBA, and fan static efficiency was determined as 100%*(CFM×static pressure)/(6,356×BHP). The brake horsepower (BHP) was measured once the fan had reached steady state operation. As illustrated in Table 1, the smallest prior art fan tested circulates air at 4100 cubic feet per minute, operates at an efficiency of 36%, and produces a sound pressure level of 82 dBA in applications requiring one inch water gauge of fan static pressure. The relatively low efficiency and high sound level of this fan leaves significant room for improvement in the industry.
TABLE 1Prior Art Fan1″2″3″CFMBHPEff.DBABHPEff.DBABHPEff.dBA41001.7836%822.6050%833.4656%8561002.5438%833.7951%855.0957%86132003.8854%766.2666%778.8071%78200005.9853%809.5766%8113.3970%82
It is further desirable for in-line centrifugal fans to be easy to install and service. For example, fans are typically installed within ductwork to circulate air throughout a building, and should be easily attachable and detachable to allow the fan to be easily serviced. Currently, additional parts are needed to install the fans, including separate angle rings and flexible duct connectors that are used to eliminate the transmission of vibration from the fan. Furthermore, servicing conventional fans' internal drive components has typically been limited and cumbersome due to the limited accessibility to their internal drive components, which requires the removal, and disassembly, of other internal components. Subsequently, the non-modular moveable parts need to be reinstalled within the fan, which is difficult given the small internal confines of the fan.
What is therefore needed is an improved mixed flow fan that produces lower sound levels during operation, and that is more efficient to operate. It is further desirable to provide such a fan that is relatively easy and efficient to install and service.